EVOLUTION OF EDWARDS (BALCONES FAULT ZONE) AQUIFER CONCEPTUAL MODELS AND RECHARGE ESTIMATES

The general conceptual of the Edwards Aquifer has stood for over a century, but there have been significant revisions as new data have emerged. The unchanged basics are that the most important source of recharge is losing streams, a few major springs (particularly Barton, San Marcos, Comal, and Hueco) provide the most discharge, and the Edwards Limestone (and equivalent formations) host the aquifer. However, numerical models of the aquifer and its recharge have evolved. Early models of the San Antonio and Barton Springs segments of the aquifer estimated recharge from a limited stream gauges and underestimated discharge to sources other than the major springs. Direct recharge was calculated as the difference between these recharge estimates and gauged spring discharge. Detailed stream gauging shows stream losses are concentrated in limited sites in the streambeds and stream loss calculation is temporally and spatial complex. Recent measurements of evapotranspiration and precipitation in recharge areas show direct recharge is more comparable to other karstic areas. Additional recharge has been shown to come from the Trinity Aquifer, smaller ungauged creeks that flow over the recharge area, and urbanization effects. Groundwater divides fluctuate spatially and some disappear during droughts. Tracer tests have redefined recharge area. Significant discharge is now known to occur also at Cold Springs in the Barton Springs segment and at Los Moras Springs and the Leona River alluvium in the San Antonio segment. In some places, faulting has juxtaposed the underlying Glen Rose and the overlying Buda Limestone and Austin Chalk so that these units are part of the aquifer. Models are very important tools for evaluating aquifers, but they must evolve conceptually and numerically to incorporate new findings. The need to manage these critical water resources requires accurate aquifer models in this area of rapid population growth.